This invention relates to photovoltaic devices of the type formed from multiple amorphous semiconductor alloy regions deposited on a substrate. The invention more particularly relates to photovoltaic devices including at least one narrow band gap amorphous semiconductor alloy region and which provide enhanced open circuit voltage.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and other type devices which are, in operation in photovoltaic and other applications, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have acceptable concentrations of localized states in the energy gaps thereof, and provide high quality electronic properties. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent To Crystalline Semiconductors, which issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374 which issued in the names of Stanford R. Ovshinsky and Masatsugu Izu, on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
Amorphous semiconductor materials are of great commercial importance because such materials enable mass production of photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in patents pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom, now U.S. Pat. No. 4,400,409, issued Aug. 23, 1983; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System, now U.S. Pat. No. 4,410,558, issued Oct. 18, 1983; and Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition And Isolation System And Method, now U.S. Pat. No. 4,438,723, issued Mar. 27, 1984. As disclosed in these patents and applications, a substrate formed from stainless steel, for example, may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material.
In making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patents and patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed toward utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Vo.sub.c). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by one or more smaller band gap materials to absorb the light passed through the preceeding cell or layer.
The tandem photovoltaic devices formed from amorphous semiconductor alloys have also shown increased utilization of the solar spectrum for generating electrical energy as compared to single cell devices. Such a tandem device and a method of making the same is disclosed for example in U.S. patent application Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. The tandem device therein disclosed, and of the type to which the present invention is particularly directed, includes a plurality of amorphous semiconductor photovoltaic gells of the p-i-n configuration disposed in series relation on a substrate. Preferably, the intrinsic regions of the cells have different band gaps which are progressively narrower from the top cell (the cell upon which the incident radiation first impinges) to the bottom cell. As a result, each cell absorbs photons in different portions of the solar spectrum for creating charge carriers and generating electrical current from the collected carriers. Because the cells are coupled in series, it is necessary to match the photon generated current of the cells. This can be achieved by adjusting the band gaps and the thickness of the cell intrinsic regions as mentioned above.
In order to properly adjust the band gap of the cell intrinsic regions, it is necessary to narrow the band gap of at least one of the intrinsic regions. When the devices are formed from amorphous silicon alloys, this requires the incorporation of one or more band gap decreasing elements into the amorphous silicon. Germanium is one such band gap decreasing element.
Germanium can be incorporated into amorphous silicon alloys by, for example, the glow discharge decomposition of germane gas (GeH.sub.4) together with a silicon containing gas such as silane (SiH.sub.4). However, it has been observed that as the germanium concentration in the alloys is increased the electrical properties become increasingly inferior. The inferior electrical properties of these alloys is primarily due to an increased density of defect states in the band gaps of tese materials. This causes amorphous silicon-germanium alloys to have short carrier lifetimes, narrower collection widths and higher dark conductivities. The electrical properties diminish to the point that when the germanium concentration is above about fifty percent (50%, resulting in an adjusted band gap narrower than about 1.4 eV, the material is no longer suitable for photovoltaic applications. One effect of the increased density of defect states in amorphous silicon-germanium alloys is the reduction in open circuit voltage by an amount larger than can be reasonably explained by the reduction in band gap. This reduction in voltage is attributed to increased recombination at the increased defect states, and to interface states introduced by band gap and structural mismatch at the doped region-intrinsic region boundary.
Applicants herein have invented a new and improved device configuration which improves the performance of photovoltaic devices incorporating amorphous silicon-germanium alloys notwithstanding the foregoing. The new and improved devices incorporating the invention demonstrate enhanced open circuit voltage (Vo.sub.c) over that previously obtainable with these narrow band gap alloys. The enhancement has been observed to be as much as 80 mV, representing an overall increase of about ten percent (10%) in device efficiency for a single cell device.